Collaborative Development of Zero Discharge Desalination Technology

ZDD (Zero Discharge Desalination) is a technology that was developed with research funding from the Bureau of Reclamation and is being commercialized by Veolia with the support of the University of Texas at El Paso. The ZDD technology is directed to reduction in the volume of concentrate produced in the desalination of groundwater that contains sparingly soluble solutes, particularly calcium sulfate. Initial research to demonstrate feasibility was funded by Sandia National Laboratories. Subsequently Reclamation funded pilot studies, first for the treatment of irrigation drainage containing calcium sulfate and selenium and second for treatment of groundwater at the Brackish Groundwater National Desalination Research Facility (BGNDRF) in Alamogordo, NM.

ZDD technology utilizes electrodialysis metathesis (EDM) to treat the concentrate produced by another desalination process, usually RO or NF. EDM devices utilize the same types of anion exchange and cation-exchange membranes as normal electrodialysis, but there are four membranes and solution compartments in the repeating unit of an EDM stack. The RO or NF is operated under conditions in which the CaSO₄ content is at a safe level. The RO/NF concentrate is then sent to the EDM stack as one of the two feed streams. The salt-depleted diluate stream from the EDM is returned to the RO/NF for recovery of more water. The other feed stream to the EDM contains NaCl. The salts in the two feed streams change partners in the EDM stack to produce two highly concentrated streams, one rich is CaCl₂ and the other rich in Na₂SO₄. Because these two salts are highly soluble, they can be concentrated to high levels. Typically the volume of each concentrate stream is about 1% of the volume of feed water to the RO/NF. These two concentrate streams from the EDM can be processed separately to recover useful compounds, or they can be combined to precipitate CaSO₄, which has potential commercial value for soli augmentation or for production of gypsum board. There are two major benefits to combining the concentrate streams. The first benefit is that the troublesome ions in the feed water are removed as a solid byproduct instead of remaining in solution. Thus, the disposal of liquid salt solution is reduced substantially. The second benefit is that the NaCl reconstituted in the supernatant of the precipitation chamber is at a fairly high concentration. The NaCl can be recovered from the supernatant and returned to the EDM stack.

The pilot study on treatment of irrigation drainage was sponsored by the Bureau of Reclamation and took place on a grape farm in the Panoche Water District near Firebaugh, CA. The irrigation drainage water contains about 450 μg/L of selenium, nine times the MCL of 50 μg/L.  RO reduced selenium below detection limits in the permeate, but the high level of CaSO₄ in the feed water limits RO recovery to 50%. Hence, RO alone is not an adequate solution to the treatment of the irrigation drainage, because disposal of the selenium-laden RO concentrate is a real problem. Treatment of the RO concentrate with EDM produced a Na₂SO₄-rich concentrate with 4700 μg/L of selenium. Thus the selenium was contained in a volume less than 10% of the original feed.

Because of natural gypsum deposits in the vicinity, the groundwater under Alamogordo, NM, is nearly saturated in CaSO₄. In some wells, notably those that would be the water source for a desalination plant for the City of Alamogordo, CaSO₄ is the dominant solute. ZDD technology is applicable for concentrate management of this water that is difficult to treat by RO alone. A yield of 98% was obtained in pilot studies of the ZDD process treating the Alamogordo groundwater.